This invention generally relates to jack-up offshore platforms of the type that include a platform structure, a plurality of support legs extending into the water down to the ocean floor, and jacking units on the platform structure for engaging such support leg to raise and lower the platform structure relative to the surface of the water. More particularly, the present invention relates to a truss leg structure for use in jack-up offshore platforms which have a plurality of mutually parallel and laterally spaced apart column members rigidly interconnected and adapted for use with a rack and pinion jacking system.
The need for additional oil, gas and other mineral resources has in recent years brought about increased activity in the exploration for and recovery of such resources from offshore locations. In order to perform the necessary exploration drilling, production drilling, and in some instances petrochemical-processing, it is necessary to provide a platform structure from which such activities can be conducted. At locations having substantial oil and gas reserves, the approach taken in recovering the minerals has been to erect a permanent platform at the proposed well site and lay pipelines between the platform and the shore to transport the oil and gas to onshore storage or processing facilities. The fixed platform is, however, for the most part limited to situations where the reservoir is large enough to amortize the investment.
As the erection of a fixed platform requires a large capital investment, and consequently the offshore hydrocarbon reservoir must be sufficiently large to justify the investment, there are a large number of smaller wells with respectable quantities of oil and gas which cannot be produced profitably. In order to improve the profitability of the small or marginal offshore reservoirs, mobile offshore platforms have been designed to permit a single platform structure to be utilized at several successive reservoirs.
There are many different jack-up platforms in use today. However, a typical jack-up platform has a buoyant hull that permits transporting of the platforms to well site and has separate support legs that project upwardly from the hull during transport. Once the platform has reached the desired location, the support legs are lowered into contact with the ocean floor and the platform is jacked up to a level above the surface of the water. When operations are finished at a particular location, the structure can be jacked down and moved to another site.
Prior art jack-up-type offshore platforms have typically utilized a trussed leg structure of triangular or other polygonal cross-sectional configurations. The leg structure comprises a plurality of cylindrical column members that are mutually parallel and spaced apart laterally to define the corners of the geometrical shape in which the leg structure is configured. The column members are interconnected by crossbracing that extends between adjacent column members, making the leg a unitary structure. Each column member has a dual rack member in the form of a heavy flat metal bar of elongated rectangular transverse cross-section mounted to it and extending parallel through the longitudinal axis of the column member. Two sets of rack teeth are provided, each extending along a different one of the two edges of the rack member.
In jack-up platforms in the prior art having this type of leg structure, it has been taught to provide for each rack member a jacking unit having a rigid frame carrying the motor and gear reduction equipment arranged to drive pinions arranged in pairs. The axes of rotation for each pair of pinions are usually mutually parallel, horizontal and spaced apart, with the pairs being lined vertically one above the other. Each pair of pinions are spaced apart by a distance that permits each pinion to be meshed with a different one of the sets of rack teeth on a rack carried by a column member.
Typical leg supported offshore platform structures with jacking apparatus of this type are disclosed in U.S. Pat. No. 3,743,247 and U.S. Pat. No. 3,606,251, both to Willke, et al.
Offshore platforms utilizing jacking systems of the rack-and-pinion type typically have three or four separate support legs of equal length. In the case of a triangular or rectangular leg configuration, the number of separate sets of jacking systems can be from a minimum of nine or as many as sixteen. It is not only quite expensive to provide such a large number of jacking systems, but it is also necessary to synchronize the operation of each jacking unit on a particular leg structure to prevent unequal vertical displacement of one portion of the leg structure relative to the remainder.
Other jack-up platform leg structures now in the prior art include those disclosed in U.S. Pat. No. 2,924,077 to LeTourneau; U.S. Pat. No. 3,183,676 to Heitkamp; U.S. Pat. No. 3,851,482 to LeTourneau et al; and U.S. Pat. No. 3,367,119 to Rybicki. The leg structure used in all of the aforementioned patents utilizes one outwardly facing rack surface on each column of the truss leg. Although single faced rack surfaces reduce the amount of material needed to construct the support leg structure, the construction cost remains great. The Rybicki patent also discloses a triangular truss-type leg provided with racks on two columns of the leg.